The serial undiluted propagation of herpes simplex viruses results in the accumulation of defective virus genome containing head to tail reiterations of limited subsets of viral DNA sequences. The HSV defective genomes are of two types: Class I defective genomes contain DNA sequences arising from the end of the S component of HSV DNA. Class II defective genomes derive their DNA sequences from the end of the S component as well as from defined locations (map coordinates 0.35-0.45) of the L component of HSV DNA. The objectives of the studies proposed in this application are (1) to clone defective virus DNA repeat units and portions thereof in bacterial plasmids. (2) to use the cloned derivatives of defective virus DNA to more finely characterize the replication origin and DNA maturation cleavage signal which we have shown to be present in defective genome repeat units. (3) to use cloned defective genomes to assess the importance of specific DNA sequences (such as RNA promoters and specific cis acting regulatory units) in determining the expression of defective virus genomes, and in controlling the capacity of defective viruses to interfere with wild type virus replication.